The present invention relates to injection molding apparatus and, more particularly, to improvements in the construction of nozzles that deliver hot melt into the mold cavities of such equipment.
It is known in the art to provide injection nozzles with two-piece tip assemblies comprising a replaceable inner tip or insert and a collar-like retainer that detachably secures the tip to the main body of the nozzle. See, for example, Gellert U.S. Pat. No. 5,299,928.
It is also known to make the retainer from a lower thermally conductive material than the tip itself so that the tip, through which the hot melt travels on its way to the mold cavity, is thermally insulated by the retainer from adjacent portions of the relatively cold mold. The ""928 patent, for example, describes constructing the tip from a highly thermally conductive material such as a beryllium copper alloy while forming the outer retainer from a much less thermally conductive material such as a titanium alloy.
While using the retainer to insulate the hot nozzle tip from proximal portions of the cold mold is helpful in increasing the thermodynamic efficiency of the apparatus, the extent of direct physical contact between the insulating retainer and the hot tip also has a direct bearing on heat loss. Because the retainer is not a perfect insulator, there is still a significant amount of heat loss from the tip to the cold mold via the retainer, particularly across regions where the tip and the retainer are in intimate physical contact with one another.
Furthermore, as the manifold block and the nozzles attached to the block heat up as they are prepared for dispensing the hot melt, and during the injection process itself, dimensional changes take place involving the nozzles. Generally speaking, while the nozzles and manifold block tend to grow or expand as they become hot, the mold remains much cooler and dimensionally stable such that the nozzles can become misaligned with the mold cavities. For example, while the center-to-center distances between gates in a multi-cavity machine remains essentially constant at all times, the center-to-center distance between the base ends of the nozzles can increase significantly as the metal manifold block expands under high heat conditions. Consequently, while the nozzles may be in perfect registration with the gates when the apparatus is cold, the base ends of the nozzles may move out of axial registration with the mold cavity as the manifold block and nozzles heat up, placing bending loads on the nozzles as their discharge ends are retained in place by surrounding portions of the mold. This obviously places undue stress on the nozzles and can lead to premature wear and fatigue, as well as having adverse effects on the proper injection of hot melt through the gate and the ability to produce a preform product having only a minimal gate vestige at the completion of the forming cycle. Furthermore, if the manifold and the mold are pulled apart for maintenance purposes or adjustment, once the discharge ends of the nozzles are released by the mold they tend to spring back into alignment with their bases, which means that the discharge ends are now out of registration with the receiving wells in the mold and cannot be reinserted into the mold until after they have been cooled down. This can result in a significant amount of downtime in an industry where it is crucially important to keep the molding apparatus in continuous productive operation as much as possible.
In accordance with the present invention, a two-piece tip assembly on an injection molding nozzle has the insulating sleeve of the retainer surrounding the nozzle tip in radially spaced relation thereto so as to form an insulating air gap between the retainer and the tip along a significant portion of the length of the tip so as to reduce heat loss from the tip to the cold mold. The bore through the insulating retainer is constricted at its outer end so as to form a collar on the retainer that is very slightly spaced from the adjacent surface of the tip when the tip is cold. However, when the tip is hot such as during injection operations, expansion and growth of the tip relative to the retainer causes the collar to tighten around the tip so as to effectively seal off the insulating air gap from hot melt that might attempt to back fill into the air gap from beyond the nozzle. Direct physical contact between the retainer and the tip is limited to only two points, i.e., the seal at the sealing collar, and the abutment at the inner end of the retainer where it overlies and engages an outwardly facing shoulder on the tip.
In addition, the present invention contemplates having the base ends of the injection nozzles swivel-mounted in the manifold block so that the nozzles can self-adjust or self-compensate as the manifold block grows and expands when heated. To this end, the manifold block is provided with a number of concave seats that matingly receive the lower halves of spherical base portions of the nozzles. Clamp-down structure attaching the nozzles to the manifold block is provided with internal concavities that matingly receive the upper halves of the spherical bases. The retainer at the discharge end of each nozzle is configured to present a laterally outermost edge that resides in close proximity to a surrounding wall portion of the nozzle-receiving well in the mold so that the discharge end of the nozzle stays properly located and registered with the gate while permitting swiveling action at the lower end. Opposing surfaces of the mold and the nozzle tip assembly are configured in such a manner as to maximize delivery of hot melt into and through the gate while minimizing the amount of excessive back fill of melt into the void area between the mold surface and the nozzle tip assembly surface.